1. Field of the Invention
This invention relates to an adhesive, an adhesive layer for electroless plating and a printed circuit board, and more particularly to an adhesive technique capable of stably providing a printed circuit board having an excellent peel strength even in wiring with higher density and pattern accuracy.
2. Description of the Related Art
Recently, miniaturization and high-speed performance of electronic equipment has been effected with the advance of electronic industry, and consequently it is demanded to have high densification and reliability through fine pattern against printed circuit board and printed wiring substrate mounting LSI.
Lately, an additive process in which an adhesive is applied onto a surface of a substrate to form an adhesive layer, and the surface of the adhesive layer is roughened and then subjected to an electroless plating to form a conductor is noticed as a method of forming the conductor onto the printed wiring substrate.
According to this method, the conductor is formed by electroless plating after the formation of the resist, so that it has a merit that conductor wiring having high density and pattern accuracy can be produced with a low cost as compared with an etched foil process conducting the formation of pattern through etching (which is known as a subtractive process).
In the additive process, there have hitherto been known a method in which fine unevenness is formed on the conductor-forming surface side of the adhesive layer through a chemical etching as a means for improving the adhesion property between conductor and adhesive layer (hereinafter referred to as peel strength). According to this method, the unevenness of the adhesive layer surface is filled with a plating metal such as a plating copper or the like, whereby the peel strength can be improved through an anchor effect based on the unevenness. Such an improvement of the peel strength through the anchor effect is generally carried out by enlarging the braking surface or by increasing the strength of the conductor metal or the heat-resistant resin matrix constituting the adhesive.
In recent additive type printed circuit boards requiring the formation of wiring with high density and pattern accuracy, it is required to make small anchors formed by surface toughening of the adhesive layer in order to precisely form the fine pattern of the resist. Therefore, this conventional technique causes a problem that as the anchor becomes small, the breaking area is also small and hence the peel strength considerably lowers.
In order to solve this problem, there is a method of increasing the strength of the conductor metal or the heat-resistant resin matrix constituting the adhesive. However, according to the inventors' experiment, it has been confirmed that the peel of the electroless plated layer forming the conductor is caused by the breakage of the heat-resistant resin matrix in the conventional printed circuit board using copper as a conductor metal and a thermosetting resin or photosensitive resin as a heat-resistant resin matrix constituting the adhesive. That is, it is noticed that the cause of decreasing the peel strength lies in the poor strength of the heat-resistant resin matrix constituting the adhesive.